#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <sys/socket.h>

#define BUF_SIZE 30

void error_handling(char *message);

int main(int argc, char *argv[]) {
    int serv_sock;
    char message[BUF_SIZE];
    int str_len;
    socklen_t clnt_adr_sz;
    struct sockaddr_in serv_adr, clnt_adr;

    if (argc != 2) {
        printf("Usage : %s <port> \n", argv[0]);
        exit(1);
    }

    serv_sock = socket(PF_INET, SOCK_DGRAM, 0);
    if (serv_sock == -1) {
        error_handling("sock() error");
    }

    memset(&serv_adr, 0, sizeof(serv_adr));
    serv_adr.sin_family = AF_INET;
    serv_adr.sin_port = htons(atoi(argv[1]));
    serv_adr.sin_addr.s_addr = htonl(INADDR_ANY);

    if (bind(serv_sock, (struct sockaddr *) &serv_adr, sizeof(serv_adr)) == -1) {
        error_handling("bind() error");
    }

    while (1) {
        // 没有连接建立和受理的过程，不能明显区分服务端和客户端
        // 但是服务端的sock一定要分配端口
        clnt_adr_sz = sizeof(clnt_adr);
        // udp有数据边界，client运行了几次sendto这里就要运行几次recvfrom接收全部数据
        str_len = recvfrom(serv_sock, message, BUF_SIZE, 0, (struct sockaddr *) &clnt_adr, &clnt_adr_sz);
        sendto(serv_sock, message, str_len, 0, (struct sockaddr *) &clnt_adr, clnt_adr_sz);
    }
}

void error_handling(char *message) {
    fputs(message, stderr);
    fputc('\n', stderr);
    exit(1);
}